Constructing targeted bi-lamellar microstructure via heat treatment for high compressive strength and plasticity in selective laser melted Ti6Al4V-5vol%TiB composite

Abstract

Bilamellar heat treatment was applied to tailor the mechanical properties of Ti6Al4V-5vol%TiB composite manufactured by selective laser melting (SLM). The bilamellar microstructure comprises primary α (αp) lamellae and transformed β (βt) lamellae, where secondary α platelets develop. The effects of annealing temperature (850–975 °C) on the bilamellar microstructure and the coarsening of TiB whiskers were studied. The higher the annealing temperature, the coarser the βt region, and the higher its fraction. The TiB whisker clusters were transformed into individual TiB whiskers with higher aspect ratios through the Ostwald ripening process. When the annealing temperature was increased to 950 °C, the TiB whiskers underwent significant coarsening accompanied by a visible reduction in quantity. Compressive testing suggested that the bilamellar treatment led to a major improvement in plasticity at the expense of an acceptable decline in yield strength. In addition, the mechanical properties of samples with bilamellar and fully lamellar microstructures were compared, with the results indicating that the former exhibited better compressive strength and plasticity. Compared with the SLM-processed Ti6Al4V alloy, the sample annealed at 950 °C exhibited 30% and 31% enhancements in the yield strength and ultimate compressive strength, respectively, with almost no loss in plasticity.